Mammalian pineal glands have been thought to contain, and perhaps to secrete an octapeptide, arginine vasotocin (AVT), in addition to melatonin. Using a sensitive radioimmunoassay (RIA) for AVT, coupled with peptide separation using HPLC, we are attempting to confirm the existence of AVT in pineals from various species, and to examine in vivo and in vitro the factors that control AVT secretion. We also are comparing these factors with those that control melatonin release. Melatonin levels are measured by RIA and/or bioassay. We have been characterizing the ranges of both compounds at various times of day and night in pineals, plasmas, and urines from adult rats exposed to normal diurnal lighting conditions. We are also studying the effects on AVT and melatonin levels of (1) normal prenatal and postnatal development, (2) exposure to continuous light or darkness, (3) blinding of animals, denervating pineals, or subjecting animals to various central nervous system lesions already known to influence melatonin secretion, and (4) giving rats drugs known to modify the functional activity of pineal noradrenergic synapses. Finally, we are exploring possible mechanisms for controlling the synthesis and release of AVT in the pineal using rat pineal organ cultures, and cell suspension systems prepared using fresh bovine pineal glands.